1. Field of the Invention
The present invention relates to a nonlinear optical element and, more particularly, to a nonlinear optical element suitably used for an optical functional element such as an optical modulator, an optical memory, an optical switch, an optical amplifier, an optical threshold value element, an optical logic operation in various fields, e.g., an optical computer, optical communication and the like, utilizing light as an information medium.
2. Description of the Related Art
Various conventional nonlinear optical elements (in general, also called an optical bistable element) as an element producing two different optical output stable states with respect to an identical light input intensity have been proposed.
FIG. 1 is a schematic view of a conventional nonlinear optical element. In FIG. 1, a nonlinear medium 21 exhibits one or both of absorption nonlinearity and dispersion nonlinearity with respect to an incident light intensity. Reflection mirrors 22 and 23 having a predetermined transmittance constitute an optical resonator including the nonlinear medium 21.
In the nonlinear optical element shown in FIG. 1, when incident light is incident from one side in the nonlinear medium 21 through the reflection mirror 22, the incident light is absorbed or dispersed by the nonlinear medium 21 and then reaches the reflection mirror 23. In this case, some light components of the incident light are transmitted, and the remaining components are reflected by the mirror 23 to be returned to the nonlinear medium 21.
In this case, when parameters such as a phase difference of light components upon reciprocal movement in the optical resonator, the reflectances of the reflection mirrors, and the like are properly selected, nonlinearity appears in light input/output characteristics.
For example, as shown in FIG. 2, when an incident light intensity I.sub.0 to the nonlinear optical element is gradually increased from 0 and exceeds a given level, a transmitted light intensity I.sub.t is immediately increased. Such characteristics are normally called differential gain characteristics.
Depending on setting of the parameters, so-called hysteresis characteristics are obtained such that different characteristics are obtained when the incident light intensity I.sub.0 is increased and decreased, as shown in FIG. 3. Note that the hysteresis characteristics are also called nonoptical bistable characteristics.
The nonlinear optical element having the characteristics described above can be widely applied to functional elements such as a memory, switch, amplification, logic operation, optical control, and the like using light as a medium.
The operation mechanism of the nonlinear medium can be largely classified into the following two mechanisms. In one operation mechanism, an electron level is changed upon incidence of light, and a refractive index or absorption coefficient is changed. In the other operation mechanism, heat is produced upon incidence of light, and a refractive index or absorption coefficient is changed. As compared to a medium utilizing an electron effect, a medium utilizing a thermal effect tends to be operated with low power but has a low operation speed and poor stability.
Contrary to this, the medium utilizing the electron effect can perform a high-speed operation but requires high operation power. In addition, since the medium utilizing the electron effect has small changes in refractive index or absorption coefficient, a margin of the nonlinear operation is small.
Normally, most nonlinear optical elements having an arrangement in which the nonlinear medium is included in the optical resonator, and optical feedback is achieved by utilizing the reflection mirrors of the optical resonator require high operation power, thus posing the most serious problem.
For example, even ZnSe or GaAs/GaAlAs which can perform an optical bistable operation at room temperature with relatively low power requires a power density of several hundreds of W/cm.sup.2 or more.
If the operation power is high, the stability of the nonlinear optical element is impaired, and diffusion in the lateral direction due to generation of thermally excited electrons frequently occurs. For example, an element resolution during parallel processing of two-dimensional information is impaired, a relaxation time is prolonged, and an element operation cycle is lowered. Furthermore, when an element is operated in cooperation with another element with low operation power, the operation power of the other element must also be attenuated, resulting in various problems, such as energy loss, heat generation in the entire system, and the like.